1. Field of the Invention
This invention relates to pneumatic safety circuits for air inflation devices. More particularly, it relates to pneumatic safety circuits that can be installed in the pneumatic circuits of tire inflating machines or tire mounting machines being built by a machine manufacturer and also can be installed on existing tire inflating machines or tire mounting machines which are generally devoid of pneumatically operated safety circuits, to retrofit them and upgrade their safety standards.
2. Description of the Prior Art
In garages and tire installation and repair shops, a supply of high pressure, "shop" air is available to operate the pneumatic portion of tire mounting machines that are used to press, lever, and wedge tires off or onto tire rims or automobile wheels. The same air supply, at the shop pressure usually in the range of 125 and 175 psi, (862 to 1207 KPa), is used to inflate the tires to the appropriate tire pressure usually in the range of 30-50 psi, (207 to 345 KPa), to insure they are in place on the wheels. In order to seat the beads of a modern tire to the rim, it is sometimes necessary to overinflate the tire temporarily, after which the tire pressure is reduced to the desired running pressure. Also, the tires often are inflated before a valve core is installed in the valve stem, to expedite build-up of pressure in the tire. With no valve core in the valve stem, tire pressure can build up extremely rapidly with shop air.
Control of pressure build-up in the tire generally is achieved by operation of a pneumatic supply valve, either manual or pedal operated, which opens and closes in the line from the shop air supply to the tire being inflated. A pressure gauge in this line indicates, when the supply valve is closed and the line connected to the tire, what the pressure in the tire is. By intermittently operating the supply valve, the build-up of pressure in the tire can be monitored, but careless or inattentive operation of the supply valve has been known to overinflate and burst tires, causing serious damage to personnel and property.
To preclude such accidents, it has been suggested to have additional switching facilities to control the shop air supply, but these facilities appear primarily to involve electrical circuitry, making them inappropriate or useless for retrofitting existing tire mounting machines which are entirely pneumatic in operation. Furthermore, the additional switching facilities generally operated by merely reducing the pressure being applied to inflate a tire, thus greatly retarding the rate of inflation and inducing operators to bypass them to expedite their work.
In a Canadian patent application Serial No. 590114 filed 3 Feb. 1989 by Michael C. Molls and corresponding U.S. application Ser. No. 316,283 filed 27 Feb. 1989, there is disclosed a pneumatically operated safety circuit which automatically shuts off the flow of high pressure air being introduced into a tire when the pressure in the tire reaches a preselected maximum pressure significantly below the high pressure air supply. The high pressure air is supplied to the tire through a three-way pilot operated pneumatic valve whose position is alternated between an inflating cycle and a gauging cycle by operation of a pair of time-delay three-way pilot operated pneumatic valves which are set to allow high pressure air to inflate the tire for a first timed period, then interrupt high pressure air to the tire and for a second timed period, generally much briefer than the first timed period, gauge the pressure in the tire and, if the tire pressure has achieved the desired maximum pressure, interrupt the inflating cycle by maintaining the three-way pilot operated pneumatic valve in the gauging position. The safety circuit required the use of two time-delay three-way valves and involves greater expense for construction than can be achieved with the pneumatic circuit of the present invention.